Re: Variac faults: what to look for

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "marc metlicka by way of Terry Fritz <twftesla-at-qwest-dot-net>" <mystuffs-at-orwell-dot-net>

I've been following this thread right along and scott gives excellent
info here.
I might add that after disassembly (very simple) the winding core should
be flushed thoroughly with "electro clean" bought at electrical supply
houses, while there get a can of "moto finish" varnish. after cleaning
the core, tape off the comutating surface carefully and apply liberal
coats of red varnish to allow to soak into the nooks and crannies. I
personally like to use 400 grit paper on a 2x4 lightly to clean com then
a light spray of lectro clean to wash grit away.
Other then this scott is spot on on future repairs.
Marc M.

Tesla list wrote:
 >
 > Original poster: "Scott Hanson by way of Terry Fritz 
<twftesla-at-qwest-dot-net>" <huil888-at-surfside-dot-net>
 >
 > A few comments about variacs, what can go wrong, and what to look for:
 >
 > Yes, variacs (variable autotransformers) can be damaged. It's been my
 > experience that many variacs that have had at least some smoke let out of
 > them show up at swap meets and on eBay, so the buyer of a used unit has be
 > careful. I would be VERY careful of buying anything that I couldn't examine
 > first-hand unless the seller provided a money-back guarantee.
 >
 > The very first "test" is to simply run the variac through its entire range
 > of rotation. There should be no rough spots or binding. If there is, this
 > is an indication of a problem which should be identified and fully
 > understood before considering the purchase of the unit.
 >
 > Probably the most common variac fault is worn-out brushes. This can be a
 > serious problem, as replacement brushes from the manufacturer are quite
 > expensive, and may be simply unavailable for very old (discontinued) units.
 > Unless the variac is almost free, avoid units with badly worn brushes.
 > Anything with less than 3mm worth of brush projecting from the brush holder
 > is very near end-of-life. Depending the variac design, sometimes the
 > rotating member that carries the brushes can be re-positioned on the shaft
 > to provide a little more contact pressure.
 >
 > The second most common fault is overheated windings, usually shown by
 > discolored or darkened enamel on the magnet wire. In very severe cases, the
 > enamel will be literally burned away or flaking off.  Variacs showing this
 > fault need to be checked very carefully at the commutator surface. A common
 > cause of burned windings is worn brushes, which permit arcing at the
 > commutator surface and erosion of the wire. Depending on severity, this may
 > be a fatal flaw, as the erosion causes the surface of a few windings to be
 > lower than the surrounding turns, and arcing will always occur as the brush
 > passes over this point, which further erodes the windings, which worsens
 > the arcing, etc, etc. This is why the initial "smoothness" test is such a
 > good indicator of variac condition. If a variac has minor commutator
 > erosion, it can be disassembled and the entire commutator surface carefully
 > dressed with a large fine-cut file (or a piece of 320 grit silicon carbide
 > paper mounted to a dead-flat backer), but you can really remove only a
 > limited amount of material. Keeping the entire commutator surface flat
 > (co-planar) is important; you can't just file away at the eroded spot.
 >
 > A subtle problem that is sometimes found in variacs that have been
 > overloaded is brushes that have been overheated. Typically, the brush is a
 > graphite composition bar that is soft-soldered into a brass housing (the
 > graphite is first copper plated to make it solderable). Gross overheating
 > will sometimes melt this solder, or overheat the brush spring causing it to
 > lose tension. Variacs with any indication of overheated & discolored
 > windings should be checked for brush and brush-holder problems.
 >
 > Less frequently seen problems are bent brush-carrier plates or bent shafts.
 > If the brush-carrier plate wobbles, brush contact pressure will vary, and
 > in extreme cases the brush will lift off the windings at some point, or
 > reduce contact pressure to the point that arcing may occur. This type of
 > problem is strictly mechanical, and can usually be corrected by
 > straightening or re-aligning the offending part.
 >
 > Finally, remember that variacs actually have TWO brushes, one which
 > contacts the windings, and the other which contacts a slip-ring concentric
 > with the shaft. The slip-ring brush is usually solid brass or
 > copper-graphite composition, and the slip ring is brass. Occasionally the
 > slip ring surface will be found to be pitted or burned, but this can
 > usually be cleaned up with 400 grit silicon carbide abrasive paper.
 >
 > Very old ('40's vintage) variacs may be found that did not use a slip ring
 > arrangement, but instead used a heavy asbestos-insulated cable that wrapped
 > around the shaft two or three turns and connected the wiper terminal to the
 > moving brush-carrier plate. I replace this cable with a piece of
 > fiberglass-jacketed high temperature aircraft wire, exercising the usual
 > cautions with handling of any asbestos-containing material.
 >
 > Regards,
 > Scott Hanson